Bare uninsulated electric wires passing through ceramic bead insulators and electric wires having an oxide film formed by anodization or electrophoretic deposition around a conductor mainly formed of aluminum, have been known as insulated electric wires used in vacuum devices and in other high operating temperatures devices.
However, manufacturing of such insulated electric wires which are made by passing bare copper wires through ceramic bead insulators, takes much time and labor, since the bare copper wire must be passed through ceramic bead insulators one by one.
By anodization or electrophoretic deposition, oxide films could be formed only around conductors mainly formed of aluminum. The insulated electric wires manufactured by such method have a rough surface and many voids in the insulating outer films. Hence, when such insulated electric wires were used in vacuum devices, it took much time to evacuate the vacuum devices, because of gases such as air adsorbed at the surface of the voids. Further, the reduced pressure was not low enough due to slow leak problems. As a result, the attained vacuum was not high enough for many purposes.
Electric wires coated by resin including fluorine such as tetrafluoroethylene are used where high heat resistance is not very important. These wires are not suitable for high operating temperatures.
Vacuum devices requiring a high vacuum are subjected to degassing by a baking process, so as to improve the evacuation efficiency.
However, when electric wires coated with a resin including fluorine, are used at a temperature of at least 260.degree. C., the resin is decomposed, generating gas and lowering the vacuum and the dielectric breakdown voltage. Therefore, the use of such electric wires is limited to applications not requiring a high heat resistance.
U.S. Pat. No. 3,222,219 (Saunders et al.), issued on Dec. 7, 1965, discloses a ceramic coated electrically conductive wire and method for making such a wire in which a good adhesion of the ceramic coating to the metal substrate is obtained by the solution of the metal oxide, formed in the initial stages of curing, by the glassy phase, to form a saturated interfacial layer of this metal oxide in the glassy phase at the metal ceramic interface. The glassy phase at the metal ceramic interface is part of the coating which also includes a crystalline phase. This combination of a glassy phase with a crystalline phase in the coating provides a good flexibility and contributes to the bonding between the oxidation resistant conductor and the ceramic coating. While chromium oxide may be contained in the glassy phase, it does not exhibit its inherent nature in this type of glassy phase forming part of the ceramic insulating coating. Such a structure does not suggest the intentional formation of a chromium oxide layer on the oxidation resistant conductor core as taught by the present invention. A re-melting temperature of the glassy phase in the ceramic coating is about 700.degree.-800.degree. C. Accordingly, the ceramic coated conductor wire disclosed in U.S. Pat. No. 3,222,219 may not be used at a high temperature above 700.degree. C.
If a ceramic insulator directly formed on the conductor core is mainly formed of copper by vapor deposition, the conductor core is not sufficiently bonded to the ceramic insulator, since the affinity between the copper and the ceramic insulator is low. The invention wants to avoid this problem.